1. Field of the Invention
The present invention relates to a hydraulic valve lash adjuster, and more specifically, to a hydraulic valve lash adjuster which is capable of storing a large quantity of oil therein and exhibits high resistance to wear.
2. Description of Related Art
The plunger of a hydraulic valve lash adjuster is provided with an internal reservoir for oil storage because oil cannot be instantly supplied from the exterior to the valve lash adjuster when the engine is started. Thus the oil stored in the reservoir provides a source of oil for use at the re-starting of the engine.
However, in a case where the valve lash adjuster is limited in length due to the design of the engine, or the valve lash adjuster has to be installed with a certain degree of inclination, the reservoir of the valve lash adjuster is unable to store an amount of oil sufficient for the re-starting of the engine. In order to eliminate such a disadvantage, it has been proposed to provide a generally cylindrical partition member within the plunger comprising an upper plunger portion and a lower plunger portion so as to increase the reservoir capacity.
Generally, the plunger of a valve lash adjuster performs rapid reciprocating movements relative to the body of the adjuster under the action of a cam during the operation of the engine. Since an inexpensive material such as carbon steel is used for manufacture of a plunger, the outer surface of the plunger has to be hardened by quenching or carburizing to avoid wear of the outer surface of the plunger due to its contact with the inner surface of the body.
Referring to FIG. 1, there is shown a conventional hydraulic valve lash adjuster comprising a cylindrical body 11 and a hollow plunger 12. The plunger 12 includes two portions, an upper plunger portion 12a and a lower plunger portion 12b. The upper plunger portion 12a is provided therein with a generally cylindrical partition member 21. The partition member 21 is inserted into the upper plunger portion 12a so as to define a space between itself and the plunger wall, and is secured at its lower end in fluid tight sealing engagement with the inner surface of the upper plunger portion 12a by means of press fitting. The upper plunger portion 12a is welded at its lower end to the upper end of the lower plunger portion 12b.
In manufacture of the plunger, the upper and lower plunger portions 12a, 12b are treated in advance by quenching or carburizing. The surface hardened portions 33 of the upper and lower plunger portions 12a, 12b at the regions to be welded are removed to facilitate welding. Then the partition member 21 is mounted in the upper plunger portion 12a before the upper and lower plunger portions 12a, 12b are welded together by, for example, projection welding. After the welding of the two portions, the plunger thus formed is incorporated into the body 11.
Due to the fact that the two members constituting the plunger are welded together after the surface hardening, the surface hardness of the portions 33 of the upper and lower plunger portions adjacent to the welded portion 31 is seriously impaired and the residual stresses generated in the portions 33 by welding can degrade resistance to wear and fatigue.
Furthermore, during the operation of the valve lash adjuster, spatter particles 32 projecting inward from the welded portion 31 may come off and fall into the reservoir 20. As a result, the operation of a check valve 14 in the form of a ball can be impaired and the plunger can no longer perform smooth reciprocating movements within the body 11.